There are many areas in the world where hydrocarbon containing formations are disposed relatively near the surface of the earth; however, these formations are relatively thin, and therefore, when penetrated by a borehole, only a small area of the borehole is adjacent to the hydrocarbon containing formation. Consequently, the production rate often is not economical. Moreover, many of these formations are in highly unconsolidated zones, thereby causing considerable sand or other undesirable material to be produced along with the hydrocarbons. Moreover, the hydrocarbons often are of a composition which is extremely viscous and therefore difficult to produce when using conventional production methods.
It is possible to drill a borehole down into the ground, turn the borehole horizontally through a pay zone, and then extend the borehole back up to the surface of the ground so that a continuous borehole extending from an inlet to an outlet is achieved. Wallace, et al U.S. Pat. No. 4,016,942; Striegler, et al U.S. Pat. No. 3,986,557; and Vann U.S. Pat. No. 4,194,577, each propose a method of directional drilling, and reference is made to these three patents as well as to the various art cited therein, for further background of this invention.
Striegler, et al completes his borehole, and thereafter he somehow or another inserts a perforated casing throughout the entire drill string. The drill string is then withdrawn from the borehole, and it is stated that this action causes a perforated casing to be left downhole in the borehole so that steam can be forced into the inlet, with production occurring through the outlet.
Vann U.S. Pat. No. 4,194,577 drills a slanted borehole which extends horizontally through a pay zone, he then cases the borehole, and completes a very long horizontal length of the casing by perforating in a downward direction with special perforating apparatus.
In forming a borehole of 10,000 feet in length, for example, the drill bit usually is about eight inches in diameter, although it could be made larger if economics were of no consideration. During most drilling operations, it is necessary to continuously turn the bit while circulating a drilling fluid through the entire borehole annulus in order to prevent sticking the drill string. Should circulation be terminated while drilling in an unconsolidated zone, there is some likelihood that circulation would be lost and the drill string stuck thereby causing abandonment of the hole.
The interior of a four and one-half inch drill string having an eight inch bit on the end thereof is extremely small; for example, on the order of two to three inches inside diameter. It is obvious that a borehole formed in the usual manner by the employment of commercially available bits and drill strings would therefore necessarily employ an extremely small casing diameter if the teachings of the Striegler, et al patent were followed.
It would be desirable to be able to form a continuous borehole which extends from an inlet, through a pay zone, and then to an outlet; and, thereafter to be able to case the borehole with commercially available casing of as large a diameter as possible; and, at the same time, be able to retrieve the entire drill string with little danger of becoming stuck downhole. A method which satisfies this desirable drilling operation is the subject of this invention.